Cast Worldwalker Helm by paying {2}{U}.
Mendicant Core triggers, causing you to pay {1} to copy the Worldwalker Helm spell.
Resolve both Worldwalker Helm spells.
Activate Urza by tapping a Worldwalker Helm, adding {U}.
Activate a Worldwalker Helm by paying {1}{U} and tapping it, creating a token copy of the Worldwalker Helm token plus an additional Map token for each Worldwalker Helm you control.
Activate Urza twice by tapping two Map tokens, adding {U}{U}.
Repeat from step 5.

Infinite blue mana

Exile your library

Cast all cards in your library without paying their mana costs

Infinite Map tokens

Infinite Worldwalker Helm tokens

Mendicant Core, Guidelight

Urza, Lord High Artificer

Worldwalker Helm

Cast Sensei's Divining Top by paying {1}.
Mendicant Core triggers, causing you to pay {1} to copy Sensei's Divining Top.
Activate the nontoken Sensei's Divining Top by tapping it, causing you to draw a card and put Sensei's Divining Top from the battlefield onto the top of your library.
Activate the token Sensei's Divining Top by tapping it.
Holding priority, activate Krark-Clan Ironworks by sacrificing the token Sensei's Divining Top, adding {C}{C}.
Resolve the token Sensei's Divining Top's ability, causing you to draw the nontoken Sensei's Divining Top from your library.
Repeat.

Infinite card draw

Infinite draw triggers

Near-infinite storm count

Sensei's Divining Top

Krark-Clan Ironworks

Cast Myr Retriever by paying {2}.
Mendicant Core triggers, causing you to pay {1} to copy the Myr Retriever spell, creating a token copy of it.
Activate Krark-Clan Ironworks by sacrificing the nontoken Myr Retriever, adding {C}{C}.
When Myr Retriever dies, it triggers, returning another artifact from your graveyard to your hand.
Activate Krark-Clan Ironworks by sacrificing the token Myr Retriever, adding {C}{C}.
When the token Myr Retriever dies, it triggers, returning the nontoken Myr Retriever from your graveyard to your hand.
Repeat.

Infinite LTB

Infinite ETB

Infinite sacrifice triggers

Infinite death triggers

Infinite colorless mana

Infinite storm count

Myr Retriever

Cast Ugin's Nexus by paying {5}.
Mendicant Core triggers, copying the Ugin's Nexus spell.
Resolve both Ugin's Nexus spells.
The token Ugin's Nexus is put into exile due to the legend rule and its replacement effect, causing you to take an extra turn after this one.
Activate Meticulous Excavation by paying {2}{W}, returning Ugin's Nexus from the battlefield to your hand.
Repeat each turn.

Infinite turns

Ugin's Nexus

Meticulous Excavation

Combos with "Mendicant Core, Guidelight"